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Design007-Jan2018

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94 DESIGN007 MAGAZINE I JANUARY 2018
tion network reduce the impact of simultane-
ous switching noise and electromagnetic radia-
tion in high-speed digital PCB designs.
Key Points
• Supply bounce is fundamentally related
to the total inductance of the current path
or shared return paths. It is the primary
cause of simultaneous switching noise and
electromagnetic radiation.
• When supply bounce occurs, the charge
that is impressed across the power delivery
path results in common-mode voltage. It
is this common-mode voltage that creates
electromagnetic emissions.
• The magnitude of the radiated peaks
can be limited by providing a very low
AC impedance path between power and
ground.
• Supply bounce interferes with the recep-
tion of the signal at the load, depending
on the noise margin, and can cause double
clocking.
• Supply bounce gets worse as the result of
increased lead inductance, capacitive load
and simultaneously switching outputs. It
also deteriorates with reduced resistive
load.
• At high-speeds, the lead inductance of an
IC package is critical. Larger packages tend
to have more lead inductance.
• A number of approaches can be imple-
mented during layout and routing of the
PCB to minimize the voltage drop, hence
supply bounce, in the power delivery path.
References
1. Barry Olney's Beyond Design column, The
Dumping Ground.
2. Understanding and Minimizing Ground
Bounce, Fairchild Semiconductor.
3. EMC and the Printed Circuit Board, Mark
Montrose.
4. Signal and Power Integrity – Simplified,
Eric Bogatin.
5. High-Speed Digital Design, Howard Johnson.
Barry Olney is managing director of
In-Circuit Design Pty Ltd (iCD),
Australia, a PCB design service
bureau that specializes in board-level
simulation. The company developed
the iCD Design Integrity software
incorporating the iCD Stackup, PDN and CPW Planner.
The software can be downloaded from www.icd.com.au.
To contact Olney, or read past columns, click here.
A method developed by the Rice lab of chemist
Matteo Pasquali allows researchers to make short
lengths of strong,
conductive fibers from small samples
of bulk nanotubes in about an hour. The work comple
-
ments Pasquali's pioneering 2013 method to spin full
spools of thread-like nanotube fibers for aerospace,
automotive, medical and smart-clothing applications.
The fibers look like cotton thread but perform like metal
wires and carbon fibers.
It can take grams of material and
weeks of effort to optimize the pro
-
cess of spinning continuous fibers,
but the new method cuts
that down
to size, even if it does require a bit of
hands-on processing.
Pasquali and lead author and
graduate student Robby Headrick reported in Advanced
Materials that aligning and twisting the hair-like fibers
is fairly simple.
First, Headrick makes films. After dissolving a small
amount of nanotubes in acid, he places the solution
between two glass slides. Moving them quickly past
each other applies shear force that prompts the bil
-
lions of nanotubes within the solution to line up. Once
the r
esulting films are
deposited onto
the glass, he peels off sections and rolls
them up into fibers.
Pasquali said the process repro
-
duces the high nanotube alignment and
high packing density typical o
f fibers
produced via spinning, but at a size suffi
-
cient for strength and conductivity tests.
Nanotube Fibers in a Jiffy